1. Field of the Invention
The present invention relates to an inductance part such as a transformer, a choke coil, and the like used for a power supply unit and, more particularly, to an inductance part provided with a plurality of windings and a magnetic core inserted through the windings.
2. Description of the Related Art
With a reduction in the weight, thickness, length and size of an electronic part, a switching power supply unit undergoes miniaturization and, accordingly, an inductance part which is a component used in the switching power unit is also required to be reduced in the thickness. Conventionally, as a transformer which is an inductance part, a sheet transformer as disclosed in Japanese Patent Application No. JP-A-62-76509 (refer to Patent Document 1) has been proposed. In general, a primary winding of a transformer used in a switching power supply has a large number of turns and draws less current, so that it is suitably used as a sheet coil. As an example of a sheet coil that has conventionally been used, there is known one disclosed in Japanese Patent Application No. JP-A-2003-142323 (refer to Patent Document 2: paragraphs [0004] to [0006]). As described in this document, in order to increase the number of turns, a multilayer structure is adopted and windings formed in a plurality of layers are connected in series.
However, a secondary winding has a low voltage and smaller number of turns but draws comparatively a larger current, so that there may occur a case where a use of only the sheet coil is not sufficient due to the limitation of output current rating. Therefore, it is often a case where a triple insulated wire having the wire surface onto which triple insulation coating has been applied is used to constitute the sheet transformer. An example of this technique is disclosed, as an improvement for the sheet transformer, in Japanese Patent Application No. JP-A-08-316040 (refer to Patent Document 3). In Patent Document 3, a tape is stuck on a secondary winding (triple insulation wire, etc.) that has been subjected to at least a single insulation coating for simultaneously achieving both fixing of the secondary winding and insulation between the secondary winding and a magnetic core.
According to a winding structure of Patent Document 3, the secondary winding contacts a sheet coil as the primary winding only at one surface, preventing the second winding and sheet coil from being tightly-coupled. In order to achieve the tight coupling between the second winding and sheet coil, there can be considered a structure in which sheet coils 1011 and 1012 which are obtained by dividing one sheet coil into two are disposed both above and below a secondary winding 102 as illustrated in a cross-sectional view of FIG. 12 partly illustrating a winding structure, or secondary windings 1021 and 1022 which are obtained by dividing one secondary winding into two are disposed both above and below a sheet coil 101 as illustrated in a cross-sectional view of FIG. 13 partly illustrating a winding structure. However, since there exists a secondary winding draw-out portion (triple insulated wire draw-out portion) 1031 in FIG. 12, secondary winding draw-out portions (triple insulated wire draw-out portions) 1033 and 1034 in FIG. 13, the thickness of a transformer is increased by an amount corresponding to the wire diameter of the draw-out portion. When reduction of the thickness of the transformer is prioritized, the secondary winding can be provided only on one side, making it impossible to achieve the sandwich structure. Thus, the coupling between the primary and secondary windings has been sacrificed.
Reference numerals 105, 1051, and 1052 in FIG. 12 and FIG. 13 each denote a spacer serving as member for achieving insulation between the secondary winding and magnetic core 104 and as a bobbin core for winding the secondary winding therearound. FIG. 12 and FIG. 13 each illustrate only one side (left side) of the cross-section of structures of the primary and secondary windings wound around the magnetic core 104. Further, in FIG. 12, a reference numeral 1031 denotes a secondary winding draw-out portion (triple insulated wire draw-out portion) at which the secondary winding 102 is drawn out with the winding start portion thereof in the lead, and reference numeral 1032 denotes a secondary winding draw-out portion (triple insulated wire draw-out portion) at which the secondary winding 102 is drawn out with the winding end portion thereof in the lead. Further, in FIG. 13, a reference numeral 1033 denotes a secondary winding draw-out portion (triple insulated wire draw-out portion) at which the secondary winding 1021 is drawn out with the winding start portion thereof in the lead, and reference numeral 1035 denotes a secondary winding draw-out portion (triple insulated wire draw-out portion) at which the secondary winding 1021 is drawn out with the winding end portion thereof in the lead. Further, in FIG. 13, a reference numeral 1034 denotes a secondary winding draw-out portion (triple insulated wire draw-out portion) at which the secondary winding 1022 is drawn out with the winding start portion thereof in the lead, and reference numeral 1036 denotes a secondary winding draw-out portion (triple insulated wire draw-out portion) at which the secondary winding 1022 is drawn out with the winding end portion thereof in the lead.